The subject matter of this application relates to apparatus for discharging material from a rotary mill that is used for grinding or comminution.
FIGS. 1 and 2 show a rotary grinding mill 1 that contains material 2 to be ground therein with the aid of grinding media. The mill 1 is arranged to rotate around a rotation axis 3. The mill has a feed trunnion 4 and a discharge trunnion 5 by which the mill is supported on bearings (not shown) to a mechanical ground. The material 2 to be ground in the mill is fed into a grinding chamber of the mill 1 through the feed trunnion 4. Water is advantageously also fed into the mill 1 in order to create a wet grinding in the mill 1. Between the grinding chamber and the discharge trunnion 5 of the mill 1, a framework 6 is installed inside the mill 1 and supported to the body 7 of the mill 1. The framework 6 supports a pulp lifter assembly that comprises guide members 8, 9 and a discharge cone 10. The pulp lifter assembly directs the ground material from the grinding chamber to the discharge trunnion 5 of the mill 1. As illustrated in FIG. 2, the pulp lifter assembly comprises several sequential pulp lifters 11. Each pulp lifter 11 is attached to a grate or screen 12 having holes 13 through which the ground material 2 passes and enters a slurry pocket of the pulp lifter. As illustrated in FIG. 1, at least one pulp lifter 11 is at least partly immersed into the material 2 at a time during the operation of the mill 1. The pulp lifter 11 has a substantially rectangular or trapezoidal external shape so that two external sides or edges 21 of the pulp lifter 11 are essentially parallel and two other external sides or edges 22 are convergent to each other. The pulp lifter 11 is installed in the mill 1 so that the longer external side of the two parallel sides 21 is radially outward of the shorter of the two parallel sides and is close to the body 7 of the mill 1.
FIGS. 3 to 5 illustrate two pulp lifters 11A, 11B partially connected to each other. Each pulp lifter 11 has a first section 15 and a second section 16 separated by a wall 23. The grate or screen 12 with screening holes 13 is installed in front of the first section 15 of the pulp lifter 11 in the proceeding direction 19 of the material. Between the first section 15 of the pulp lifter 11B and the second section 16 of the pulp lifter 11A there is an opening 17. The second section 16 of each pulp lifter 11 is provided with a guide member 18, which extends from a point in the vicinity of the radially outer end of the leading edge 22 of the pulp lifter (with respect to the direction of rotation 24 of the mill) to a point in the vicinity of the radially inner end of the trailing edge 22 of the pulp lifter. As shown in the drawings, the guide member is constructed so that at least the part starting from the inlet of the second section is curved over at least 25% of the total length of the guide member. The outer end of the guide member (or the leading end in the direction of rotation of the mill) is directed tangentially of the mill whereas the inner or trailing end is directed essentially towards the rotating axis 3 of the mill 1.
During the operation of the mill 1, the mill 1 is rotated around its rotation axis 3 and the pulp lifters 11 are one after another immersed into the ground or comminuted material 2. While a given pulp lifter (such as the pulp lifter 11A) is immersed, some of the material 2 flows through the sieve or screen 12 into the first section 15 of the pulp lifter 11A. As the mill 1 continues to rotate, the first section 15 is step by step lifted from its immersed state, and the material in the first section 15 of the pulp lifter 11A flows downward into the second section 16 of the pulp lifter 11B through the opening 17. Owing to the guide member 18 in the second section 16 of the pulp lifter 11B the material flow is directed towards the center of the mill 1 and further by means of the guide members 8, 9 and 10 into the discharge trunnion 5 of the mill 1 and to the further processing of the material 2.
As the pulp lifter 11A rises, material that is in the radially outer region of the first section 15 flows downwards (see the arrow 19 in FIG. 4) into the second section 16 of the pulp lifter 11B through the opening 17 and is directed towards the central axis of the mill by the guide member. As the pulp lifters continue to rise, the material in the section 16 of the pulp lifter 11B is further directed towards the central axis and is discharged from the pulp lifter onto the guide members 8 and 9, which direct the material onto the cone 10. The material is unable to accumulate or collect in the outer lower corner region of the section 16.
The mill shown in FIGS. 1-5 rotates in the counter clockwise direction as seen in FIG. 2. Let us consider the situation where the pulp lifter 11A is at the 6 o'clock position (directly below the axis of rotation of the mill). In this case, several holes 13 in the grate 12 are immersed in the slurry and slurry enters the first section 15 of the pulp lifter 11A. Slurry also flows through the opening 17 into the second section 16 of the pulp lifter 11B, but cannot enter the lower rear (outer trailing) corner region of the second section because that region is blocked by the guide member 18. As the mill rotates from the 6 o'clock position towards the 3 o'clock position, the orientation of the pulp lifter 11A changes and some of the holes in the forward rows are exposed above the slurry while at least the radially outermost hole of the trailing row remains immersed. Since the slurry on the upstream side of the grate and the slurry in the first section 15 are in communication, pressure equilibrium between the upstream side of the grate and the first section is attained if the slurry in the first section of the pulp lifter flows downwards as the pulp lifter 11A rises, so that the free surface of the slurry in the pulp lifter tends to remain always lower than the free surface of the slurry on the upstream side of the grate keeping the flow gradient across the grate. In case the mill is fed more material 2 than the designed capacity of the pulp lifters, there is a possibility that some slurry will flow back out of the first section to the upstream side of the grate, but because the opening 17 is much larger than the holes 13 the major effect will be that the equilibrating flow will pass through the opening 17 into the second section 16 of the pulp lifter 11B. Further, because of the curved shape of the guide member, the lowest point in the available space in the second section 16 of the pulp lifter 11B, i.e. the space that is not blocked by the guide member 18, will move radially inwards, towards the central axis of the mill, as the mill rotates from the 6 o'clock position towards the 3 o'clock position instead of remaining in the lower outer corner of the second section. Depending on the depth of the slurry on the upstream side of the grate, some of the slurry in the second section may overflow the radially inner end of the guide member 18 and move towards the guide cone 10. In any event, when the pulp lifter 11A reaches the 3 o'clock position substantially all the slurry will have passed into the second section of the pulp lifter 11B and much of the slurry will have moved from the pulp lifter 11B towards the guide cone and as the pulp lifter reaches the 12 o'clock position, slurry will fall downward from the pulp lifter onto the guide cone 10.
FIG. 6 illustrates a practical implementation of the pulp lifter that is shown more schematically in FIGS. 3-5. Viewing the pulp lifter along the axis of rotation of the mill, the pulp lifter has a continuous back wall 24, an inner edge wall 25 formed with a discharge opening (not shown), and a leading edge wall 26. The pulp lifter is open at its front side. An intermediate wall 23 is spaced from the back wall 24 and is connected to the back wall by the guide 18. The guide 18 and the intermediate wall 23 separate the first section 15 of the pulp lifter from the second section 16. The leading edge wall 26 is formed with transfer openings 17. The grate (not shown) is attached to the pulp lifter using fasteners that engage holes 27 in the leading edge wall. When multiple pulp lifters are installed in a grinding mill, the first section 15 of the leading pulp lifter communicates with the second section 16 of the following pulp lifter through the transfer openings 17 in the leading edge wall 26 of the following pulp lifter. In operation, slurry enters the first section 15 of a pulp lifter through the holes in the grate as the lifter passes through the 6 o'clock position. As the pulp lifter rotates towards the 3 o'clock position, the pulp lifter rises relative to the following pulp lifter and slurry in the first section 15 of the leading pulp lifter flows through the transfer openings 17 into the second section 16 of the following pulp lifter. As the pulp lifters continue to rotate, the slurry in the second section of the following pulp lifter flows along the guide 18 and flows through the opening in the inner edge wall 25 towards the cone 10, as explained above. The configuration of the guide 18 is somewhat different in FIG. 6 from FIGS. 3-5, in that the radially outer end of the guide is not tangential to the periphery of the mill, but the essential function of the guide, preventing comminuted material from remaining against the peripheral wall of the mill as the pulp lifter rotates from the 6 o'clock position towards the 3 o'clock position, is the same.
FIGS. 7 and 8 illustrate another pulp lifter. The pulp lifter shown in FIGS. 7 and 8 is similar to that shown in FIG. 6 except that the intermediate wall 23 is not coextensive with the back wall 24 but extends only over the second section 16 of the pulp lifter. Thus, the space between the back wall and the intermediate wall that is not available to slurry in the lifter shown in FIG. 6 because of the guide 18 is part of the first section in the lifter shown in FIGS. 7 and 8.
Consequently, the area available for transfer of slurry from the first section 15 to the second section 16 via the transfer opening 17 is greater in the case of FIGS. 7 and 8 than in the case of FIG. 6. In addition, it will be appreciated that when multiple pulp lifters as shown in FIG. 6 are installed, the trailing edge wall 28 of the leading pulp lifter partially blocks the transfer openings 17 of the following pulp lifter, and only the portion forward of the dashed line 29 shown in FIG. 6 is available for flow of slurry. In the case of FIGS. 7 and 8, for a pulp lifter of similar size the transfer openings 17 of the following pulp lifter are of greater effective area because they are not partially blocked by the leading pulp lifter.
The use of the guide 18 in the pulp lifters shown in the drawings is advantageous for several reasons. First, the transfer of slurry from the first section 15 to the second section 16 through the transfer opening prevents flowback through the grate from the second section as the pulp lifter rises from the 6 o'clock position to the 3 o'clock position. Second, by preventing accumulation of material in the outer trailing area of the pulp lifter, the guide 18 ensures that there is minimal carryover of pebbles and slurry as the mill rotates.
The pulp lifter assembly described in U.S. Pat. No. 7,566,017 includes a pulp lifter structure that comprises an outer pulp lifter, an inner pulp lifter, and a discharger. Referring to FIGS. 9-13 of the drawings, in which the pulp lifter structure is oriented so that it rotates in the clockwise direction when viewed along the axis of rotation of the mill from the feed trunnion, the outer pulp lifter has a leading wall 102, a radially outer wall 104, a radially inner wall 106, an axially downstream wall 108, and an intermediate wall 110 that is generally parallel to and spaced from the axially downstream wall 108 and is connected to the axially downstream wall by a curved guide 112. The walls 102-110 and the guide 112 define an inlet chamber 115 that is open towards the viewer and to the right of the figure. The leading wall 102 is formed with a transfer opening 117 (FIG. 9A) that provides access to an outlet chamber 116 defined between the intermediate wall 110 and the axially downstream wall 108 and bounded by the guide 112. The radially inner wall is formed with an outlet opening 119. Multiple outer pulp lifters as shown in FIGS. 9 and 9A are attached to the axially downstream wall of the mill in an annular array. The inlet chamber 115 of a leading pulp lifter communicates with the outlet chamber 116 of a following pulp lifter via the transfer opening 117 in the wall 102 of the following pulp lifter.
Referring to FIG. 10, inner pulp lifters 120 are attached to the axially downstream wall of the body of the mill in an annular array inward of the outer pulp lifters 100. There is one inner pulp lifter 120 for each two adjacent outer pulp lifters 100. Each inner pulp lifter 120 comprises an axially downstream wall 122 and two radial walls 124, the radial walls 124 being aligned respectively with the leading walls 102 of two adjacent outer pulp lifters 100. Each two adjacent radial walls 124 of an inner pulp lifter define a channel 126 into which the outlet opening of an outer pulp lifter debouches. Similarly, the following radial wall 124 of a leading inner pulp lifter and the leading radial wall of a following inner pulp lifter define a channel into which the outlet opening 119 of an outer pulp lifter debouches.
The pulp lifter structure further comprises dischargers 130 (FIGS. 11 and 12) that are attached to the axially downstream wall of the mill in an annular array inward of the inner pulp lifters 120. Each discharger has an axially downstream wall 132 and two radial walls 134 and 136 projecting from the wall 132. Each discharger defines a discharge channel between its two radial walls 134, 136, and each two adjacent dischargers define a discharge channel between the following wall 136 of the leading discharger and the leading wall 134 of the following discharger. It will be noted from FIG. 11 that the leading wall 134 is radially shorter than the following wall 136. The channel defined between the two walls 134, 136 of the discharger, and the channel defined between the wall 134 of the leading discharger and the wall 136 of the following discharger, open into a discharge space defined between the wall 136 of the leading discharger and the wall 136 of the following discharger. The axially downstream wall 132 of the following discharger is formed with an opening 138 that communicates with the discharge space defined between the following wall 136 of the following discharger and the wall 136 of the leading discharger.
Referring to FIG. 12, a center liner 140 is attached to the inner pulp lifter 120 and a grate plate 150 is attached to the outer pulp lifter 100. The grate plates 150 collectively form the grate of the grinding mill.
In operation, as the mill rotates and an outer pulp lifter approaches the 6 o'clock position, slurry (which may include pebbles) enters the inlet chamber through the openings 152 in the grate plate. As the outer pulp lifter moves towards the 9 o'clock position, the outer pulp lifter rises relative to the following pulp lifter and slurry in the inlet chamber 115 of the leading pulp lifter flows through the transfer opening 117 in the leading wall of the following outer pulp lifter and enters the outlet chamber 116 of that pulp lifter. As the mill continues to rotate, the slurry in the outlet chamber of the outer pulp lifter flows along the guide 112 and flows through the opening 119 in the radially inner wall 106 into the channel 126 of the inner pulp lifter, and ultimately into the discharger 130. Most of the slurry leaves the discharger through the opening 138 and moves towards the guide cone (not shown).
The speed with which particles in the pulp lifter move towards the dischargers 130 influences the efficiency of the pulp lifter structure, in that higher velocity particles are likely to reach the discharge space by the time that the discharger attains the 12 o'clock position, whereas lower velocity particles are more likely to be impeded by friction against the trailing wall that bounds the discharge channel of the inner pulp lifter or discharger 130, so that the particles do not reach the discharge space by the time the discharger attains the 12 o'clock position, and are more likely to be carried over and remain in the pulp lifter structure during the next revolution of the mill.
The velocity that is attained by particles moving towards the discharger 130 depends on the curvature of the guide 112 and the angular extent of the guide about the axis of rotation of the pulp lifter structure. For larger values of the curvature of the guide, a particle moves with greater velocity radially inward along the guide as the pulp lifter rises. Similarly, for larger values of the angular extent of the guide about the axis of rotation of the pulp lifter, the particle is subject to the influence of the guide over a greater proportion of the revolution of the pulp lifter. However, ease of fabrication of the components of the pulp lifter structure, and ease of assembly, are facilitated if the pulp lifter has a smaller angular extent about the axis of rotation. The pulp lifter structure described with reference to FIGS. 9-12 is designed such that there are 32 individual pulp lifters distributed about the axis of rotation of the mill. Consequently the guide 112 of each pulp lifter has an angular extent of 11.25°. It would be desirable to increase the angular extent of the guide if this could be achieved without adversely affecting the manufacturability of the pulp lifter structure.